Dockerfile: Add torsocks binary.

This has two purposes:
1) The 'torify' wrapper is already included, and it needs torsocks.
2) Use torsocks for health checking your onion service like this:

healthcheck:
  test: ["CMD-SHELL", "torsocks nc -z <onion address> <port> || exit 1"]
  interval: 2m

docker-compose.build.yml

@@ -9,3 +9,4 @@ services:
       context: .
       args:
         tor_version: $TOR_VERSION
+        torsocks_version: $TORSOCKS_VERSION